The present invention relates to steam turbines and, more particularly, to a sealing arrangement for preventing steam leakage into an area between inner and outer glands coupled to a steam turbine shaft.
Steam turbines generally includes a shaft to which is attached a plurality of stages of blading which react against steam introduced into the turbine to cause the shaft to rotate. The shaft and blading are located within a casing or cylinder which confines the steam. At each end of the casing where the shaft exits, there are provided seals in close proximity to the shaft for preventing steam leakage from the casing. The and for preventing air inleakage into the casing. The seals in proximity to the shaft are sometimes referred to as labyrinth seals and are supported in structural elements referred to as glands. An inner gland provides the first transition between the casing and the shaft and also between the steam environment inside the casing and the outside atmosphere. An outer gland supports a second labyrinth seal and provides a final transistion to atmosphere.
The inner gland is usually spaced radially from adjacent edges of the casing around the primary extent of the turbine shaft. However, both the inner and outer glands are normally formed in two circumferential arcs of approximately 180 degrees in extent and bolted together when installed on the turbine shaft. At the bolting points, the glands are attached to the casing so that they are not free to rotate. Over the remaining extent of the inner gland, there is provided an annular seal ring which isolates the space between the inner gland and casing from the interior of the turbine. In some high pressure (HP) steam turbines, superheated steam is introduced through an aperture in the casing into the space between the inner gland and casing and then through apertures in the inner gland into the area of the gland adjacent the turbine shaft. This cylinder end wall steam is used to minimize horizontal joint distortion by heating the casing or cylinder end wall and the inner gland. It is desirable to confine this superheated steam to the cylinder end wall and the inner gland area and not allow it to leak into the outer gland area.
In order to confine the cylinder end wall steam to the cylinder or casing end wall and the inner gland, there is provided an annular finger seal between the inner gland and the casing along the outside direction of the spacing therebetween. Thus annular finger seal lies in a pair of opposed grooves formed in the casing end wall and the inner gland. This finger seal has been found to be subject to erosion from condensation of the superheated steam resulting in undesirable leakage of the steam into the area between the inner and outer glands. Thus, it is desirable to provide a method and apparatus for preventing superheated steam leaking past the finger seal from getting into the area between the inner and outer glands.